1. Field of the Invention
The present invention relates to a band correction apparatus and, more particularly, to a signal correction apparatus for correcting the frequency characteristics of band-limited signals.
2. Description of the Background Art
The band correction apparatus for the VoIP (Voice over Internet Protocol) technique, which has recently come into widespread use, packetizes voice signals into IP (Internet Protocol) packets to integrate voice with data. This integration contributes to lowering the network or communications cost. This contribution has led to the coming into widespread use of the technique.
The traditional public switched telephone network (PSTN) attaches importance to how voice signals are to be transmitted. The voice communication has used the band not higher than 3.4 kHz and hence the network is designed to establish a bandwidth per channel of 3.4 kHz. The digital transmission network is based on a communication unit of 64 Kb/sec with a sampling frequency of 8 kHz.
On the other hand, with recent widespread use of the broadband technique and services, the transmission equipment on the network side is now designed to cope with broadband communications. Moreover, even the subscriber lines are coping with the broadband network by means of asymmetrical digital subscriber lines (ADSL) or optical transmission lines, such that end-to-end broadband voice signal transmission has become possible. Currently, there is a demand for voice communications to a still higher quality.
However, with existing general subscriber telephone sets, which are not IP-dedicated telephone sets adapted for IP network, the bandwidth is limited by e.g. an attenuator to 4 kHz or less for setting telephone transmitter and receiver characteristics. With such existing general subscriber telephone sets, the speech quality reached is no more than substantially the same as that allowed by the public switched telephone network in general, even though the transmission line allows frequency band signals higher than 4 kHz as in the IP network.
In order to solve this problem to achieve a meritorious high speech quality even when the existing telephone set uses a transmission channel, such as IP network, which employs broadband signals in common, such a method is currently researched which consists in correcting the frequency characteristics of telephone transmitting and received signals to expand the speech band.
Meanwhile, with the speech band expanding apparatus and method disclosed by the Japanese Laid-Open Patent Publication No. 2002-82685, if a high frequency range signal, which inherently does not exist, is formulated using a low frequency range signal, a speech sound formulated is heard severely unnatural as compared to an original speech sound. Moreover, only a minor amount of speech signal is left in a frequency range lower than the usual range and in a frequency range higher than the usual range. Thus, a calling party is unable to perceive these voice signal components, so that the speech sound, as heard by the calling party, is only of inferior sound quality. In improving the sound quality, the minor signals left in the respective bands may be amplified, as a simple method. However, this simple band expansion is achieved by re-amplifying the components of the lower and higher ranges to raise these components to an audible level. Consequently, the amplification of the band components in the audio signal directly leads to amplitude amplification, such that the amplitude tends to exceed the limitations on the maximum and minimum values in the digital signal processing.
The conventional sequence of operation in expanding the band will now be described. The signal limited in the bandwidth to the range of 300 Hz to 3.4 kHz is supplied as an input signal to a correction filter. The correction filter, responsible for expanding the frequency band, has such characteristics that the band of 300 Hz to 3.4 kHz is not amplified, with the amplification factor by the filter being then unity, while the frequency bands of 0 Hz to 300 Hz and of 3.4 kHz to 8 kHz are amplified by respective amplification characteristics. By this correction, the correction filter outputs a signal having flat characteristics for the frequency range of 0 KHz to 8 kHz.
However, the impulse response, representing the overall filter characteristics, has an amplification degree of +30 dB. In particular, the amplification degree is +40 dB insofar as solely the high frequency range is concerned. It is now assumed that the μ-low is used. If a signal having a magnitude not less than −27 dBm0 is supplied, the output signal is clipped because its instantaneous value readily reaches the maximum value. If the input signal is an ideally bandwidth-limited one, no severe problem is raised. However, if there is an electrical noise outside the range of 300 Hz to 3.4 kHz, this noise is also amplified by +30 dB to +40 dB. For example, with an assumed level of the substrate noise of −50 dBm0, the amplified substrate noise reaches −20 dBm0 to −10 dBm0.